NCERT Solutions Curiosity Chapter 11 End-of-chapter questions — Keep the curiosity alive

Book page 187 Updated on2026-09-05

Q1.

State whether the following statements are True or False. (i) We can only see that part of the Moon which reflects sunlight towards us. (ii) The shadow of Earth blocks sunlight from reaching the Moon causing phases. (iii) Calendars are based on various astronomical cycles which repeat in a predictable manner. (iv) The Moon can only be seen at night.

Answer

Statement	True / False	Reason
(i) We can only see that part of the Moon which reflects sunlight towards us.	True	The Moon emits no light of its own. Only the portion that is both lit by the Sun and turned towards the Earth can be seen.
(ii) The shadow of Earth blocks sunlight from reaching the Moon causing phases.	False	Phases come from the changing Sun–Earth–Moon angle as the Moon revolves. Earth's shadow on the Moon causes a lunar eclipse, which happens only on some full Moon days.
(iii) Calendars are based on various astronomical cycles which repeat in a predictable manner.	True	The day, the month and the year come from Earth's rotation, the Moon's cycle of phases and Earth's revolution.
(iv) The Moon can only be seen at night.	False	The Moon rises about 50 minutes later each day, so on many days it is above the horizon in daylight — as Meera saw at the kite festival.

Tip: Statement (ii) is the single most common wrong idea about the Moon. If Earth's shadow caused the phases, we would get one every month and the shadow's edge would always be a circular arc of Earth's own size — neither of which we observe.

Q2.

Amol was born on 6th of May on a full Moon day. Does his birthday fall on the full Moon day every year? Explain your answer.

Answer

No. His birthday returns on 6 May every year, but the full Moon does not.

One Gregorian year = 365 days
12 cycles of the Moon's phases = 12 × 29.5 = 354 days
Difference = 365 − 354 = about 11 days

Why it happens: A year after his birth, the Moon has completed 12 full cycles in 354 days and then run on for 11 more days. So on 6 May of the next year the Moon is about 11 days past full — a waning crescent, close to new Moon. The date on the wall calendar is fixed by the Sun; the phase is fixed by the Moon; the two cycles simply do not divide into each other.

Did you know? They do come back together, but slowly. 235 lunar months (about 6940 days) are almost exactly 19 solar years (also about 6940 days), so the same phase returns to the same date roughly every 19 years. Amol can expect a full Moon birthday at ages 19, 38 and 57.

Q3.

Name two things that are incorrect in Fig. 11.10.

Answer

Two things in that picture cannot happen:

Stars are shown inside the dark part of the Moon. The Moon is a solid, opaque rock about 3500 km across. Anything behind it is hidden, so no star can ever appear within the Moon's disc. Stars can be drawn all around the Moon, never on it.
The non-illuminated part of the Moon is shown as a visible dark disc. We can see only the portion that reflects sunlight to us. On a crescent night the rest of the Moon is not black against the sky — it is simply not seen at all, and the sky and its stars appear right up to the edge of the bright crescent.

Left: the dark half drawn as a visible disc with stars shining through it. Right: what the sky actually looks like — the crescent alone, with the stars outside the Moon.

Check it yourself: On the next crescent night, look carefully. You may faintly see the rest of the disc glowing — that is earthshine, sunlight reflected off the Earth onto the Moon's night side. Even then, no star ever shows through it.

Q4.

Look at the pictures of the Moon in Fig. 11.11, and answer the following questions. (i) Write the correct panel number corresponding to the phases of the Moon shown in the pictures above. [Three days after New Moon; Full Moon; Three days after Full Moon; A week after Full Moon; Day of New Moon] (ii) List the picture labels of the phases of the Moon that are never seen from Earth. Hint: You can use your observations from Activity 11.1 or Fig. 11.2 as reference.

Answer

(i) Match each picture by asking one question: what fraction of the disc is bright? Three days is about one-fifth of a fortnight, and a week is half of it.

Picture label	Phase of Moon	Why
D	Three days after New Moon	A thin crescent — only about a tenth of the disc is bright, and it is growing (waxing)
E	Full Moon	The whole disc is bright
A	Three days after Full Moon	Almost full, with a thin dark crescent bitten out of one edge — a gibbous Moon that has just begun to wane
C	A week after Full Moon	Exactly half bright with a straight dividing line — the Moon is now 90° from the Sun
B	Day of New Moon	Completely dark — only the non-illuminated half faces the Earth

(ii) Picture F is never seen from the Earth.

The line dividing light from dark always joins two opposite points on the Moon's edge and cuts the disc into exactly two pieces. In F the dark part is a leaf-shaped patch lying inside the disc with lit Moon on either side of it — no arrangement of Sun, Earth and Moon can produce that.

Why it happens: Sunlight lights one complete half of the Moon, so the boundary between the lit and unlit halves is a circle running right round the Moon through its two poles. Seen from Earth it can look like a straight line (half Moon) or a curve bulging one way (crescent) or the other (gibbous), but its two ends must always land on opposite points of the Moon's rim. That is exactly the rule picture F breaks.

Q5.

Malini saw the Moon overhead in the sky at sunset. (i) Draw the phase of the Moon that Malini saw. (ii) Is the Moon in the waxing or the waning phase?

Answer

(i) She saw a half Moon — exactly half the disc bright, with a straight dividing line, and the bright half turned towards the setting Sun in the west.

Overhead at sunset means the Moon is a quarter of the sky — 90° — away from the Sun, and a 90° separation always shows a half Moon.

(ii) It is in the waxing phase (Shukla Paksha).

Why it happens: The chapter tells us the mirror case: when the bright part has shrunk to a half circle, the Moon is overhead at sunrise — that is the waning half Moon. Overhead at sunset is the opposite half of the month. It also fits the rule of thumb on page 173: a waxing Moon is easiest to spot at sunset, a waning Moon at sunrise. Over the next week its bright part will keep growing until, at full Moon, it rises just as the Sun sets.

Q6.

Ravi said, “I saw a crescent Moon, and it was rising in the East, when the Sun was setting.” Kaushalya said, “Once I saw the gibbous Moon during the afternoon in the East.” Who out of the two is telling the truth?

Answer

Kaushalya is telling the truth. Ravi cannot be right.

Ravi: A Moon rising in the east at the very moment the Sun is setting in the west is 180° away from the Sun. At 180° the entire lit half faces us — that is a full Moon, not a crescent. A crescent is only about 30°–45° from the Sun, so it can never be on the opposite horizon. What Ravi describes is geometrically impossible.
Kaushalya: A gibbous Moon is roughly 135° from the Sun. In the afternoon the Sun is in the western half of the sky, so a point low in the east is about that far from it. A waxing gibbous Moon does indeed rise in the east in the mid-afternoon — the chapter itself notes moonrise around 2:00–4:00 p.m., which is why the Moon can be spotted in daylight.

Why it happens: Phase and position are locked together. Once you know how much of the Moon is lit, you already know how far it must be from the Sun in the sky, and therefore roughly when it rises. Full Moon rises at sunset; the waxing gibbous rises a few hours before sunset; the waxing crescent rises in the late morning and is seen low in the west just after sunset.

Q7.

Scientific studies show that the Moon is getting farther away from the Earth and slower in its revolution. Will luni-solar calendars need an intercalary month more often or less often?

Answer

Less often.

At present: 12 lunar months = 12 × 29.5 = 354 days
Shortfall from the solar year = 365 − 354 = 11 days per year
An extra month is needed when the shortfall adds up to about 29.5 days
Time taken = 29.5 ÷ 11 ≈ 2.7 years — the chapter's "every 2–3 years"

Suppose the Moon slowed until one cycle took 30 days:
12 lunar months = 12 × 30 = 360 days
Shortfall = 365 − 360 = only 5 days per year
Time taken = 30 ÷ 5 = 6 years

Why it happens: A slower Moon means a longer cycle of phases, so 12 lunar months come to more days than they do now and the lunar year sits closer to the solar year. The annual shortfall — the whole reason an Adhika Maasa exists — becomes smaller, so it takes many more years to build up to a full month. The correction is then needed less frequently.

Did you know? This is a real, measured effect. Laser reflectors left on the Moon by lunar missions show that it is moving away from us by about 4 cm a year, and as it moves out its revolution slows.

Q8.

A total of 37 full Moons happen during 3 years in a solar calendar. Show that at least two of the 37 full moons must happen during the same month of the solar calendar.

Answer

Count the months available and compare them with the number of full Moons.

Months in 3 years of a solar calendar = 3 × 12 = 36 months
Number of full Moons = 37
37 is greater than 36

Suppose, on the contrary, that no two full Moons shared a month. Then each of the 37 full Moons would need a month of its own, so we would need at least 37 different months. But only 36 months exist in those three years. That is impossible, so the supposition is wrong: at least one calendar month must contain two full Moons.

Why it happens: The reasoning is the pigeonhole idea — if you place 37 objects into 36 boxes, some box must hold more than one. The astronomy behind it is that a lunar cycle of 29.5 days is shorter than almost every calendar month, so full Moons arrive slightly faster than months do and one is bound to be caught twice. Over 3 years there are 3 × 365 = 1095 days, giving 1095 ÷ 29.5 ≈ 37 full Moons — exactly the number in the question.

Did you know? A second full Moon within one calendar month is popularly called a "blue Moon". Nothing about it is blue, and it is a fact about our calendar rather than about the Moon.

Q9.

On a particular night, Vaishali saw the Moon in the sky from sunset to sunrise. What phase of the Moon would she have noticed?

Answer

She would have seen the full Moon (Purnima).

Why it happens: To be above the horizon for the whole night, the Moon must rise exactly as the Sun sets and set exactly as the Sun rises. That is only possible if the Moon is directly opposite the Sun in the sky — a separation of 180°. At 180° the Earth is looking at the Moon from almost the same direction as the sunlight, so the entire illuminated half is turned towards us: a full bright circle. This is position A, Day 1 in Fig. 11.5.

Any other phase would fail. A half Moon is only 90° from the Sun and so is up for only about half the night; a crescent is close to the Sun and sets soon after it, or rises just before it.

Q10.

If we stopped having leap years, in approximately how many years would the Indian Independence day happen in winter?

Answer

About 700 to 750 years — roughly seven centuries.

Seasons repeat in about 365¼ days; a calendar with no leap years counts only 365 days
Drift each year = ¼ day = 0.25 day
So the calendar slips 1 day behind the seasons every 4 years

15 August is now in the rainy season. Winter weather comes about 6 months earlier in the seasonal cycle:
Days to slip = 365 ÷ 2 ≈ 183 days
Years needed = 183 × 4 = about 730 years

Why it happens: Without the extra day, each calendar year ends about six hours before the Earth has actually finished its orbit. The calendar therefore runs slightly fast, and a fixed date such as 15 August arrives a little earlier in the seasonal cycle each year. After roughly 730 years it would arrive where mid-February stands today — a winter date. It would take about 4 × 365 ≈ 1460 years for the seasons to come all the way back to where they started.

Tip: This is precisely the problem the leap year was invented to prevent, and why the Gregorian calendar goes further and skips leap years in 1700, 1800 and 1900 while keeping them in 1600 and 2000. Those small corrections keep the seasons pinned to their dates for thousands of years.

Q11.

What is the purpose of launching artificial satellites?

Answer

Artificial satellites are launched because a machine placed in orbit can see, hear and reach far more of the Earth than anything on the ground can.

Purpose	What it does for us
Communication	Carries telephone, television and internet signals between places too far apart to be linked directly
Navigation	Lets a receiver on the ground, in a ship or in an aircraft fix its exact position
Weather monitoring	Tracks clouds, storms and cyclones from above, so warnings can be issued in time
Disaster management	Maps floods, cyclones and earthquakes so relief can be sent where it is needed
Scientific research	Studies the Earth, the Sun, the planets, stars and other celestial objects from above the atmosphere

India's own missions show each of these at work. Cartosat takes high-quality images of the Earth to improve maps, plan cities and handle natural disasters, and the Bhuvan platform uses them to show terrain, soil, land use and vegetation. AstroSat observes stars and other celestial objects. Chandrayaan 1, 2 and 3 went to the Moon, Aditya L1 studies the Sun and Mangalyaan went to Mars. ISRO also lets Indian students build and launch small satellites such as AzaadiSat, InspireSat-1 and Jugnu.

Did you know? After their useful life these satellites and their rocket parts become space junk. Small pieces burn up as they fall through the atmosphere, but large ones can reach the ground and any of them can collide with a working satellite — so countries are now working together to remove this debris.

Q12.

On which periodic phenomenon are the following measures of time based: (i) day (ii) month (iii) year?

Answer

Measure of time	Periodic phenomenon it is based on	Length
(i) Day	The rotation of the Earth about its own axis, which makes the Sun return to its highest point in the sky — found from the shortest shadow	24 hours (the mean solar day)
(ii) Month	One complete cycle of the phases of the Moon, caused by the Moon's revolution around the Earth	About 29.5 days
(iii) Year	The revolution of the Earth around the Sun, which gives one complete cycle of seasons	About 365¼ days

Why it happens: Each unit is one turn of a different wheel — the Earth spinning, the Moon going round the Earth, and the Earth going round the Sun (Fig. 11.8). Because none of the three wheels turns a whole number of times while another turns once, calendars have to be adjusted: leap days for the year, and an Adhika Maasa for luni-solar months.

Was this helpful? Report an error